A technique, so called as a dye thermal transfer method, is known, in which an ink sheet containing a thermal diffusive colorant capable of thermally transferring by diffusion is faced to an image receiving layer of a thermal transfer image receiving sheet and the thermally diffusive colorant is imagewise transferred by a thermal printing means such as a thermal head or a laser to form an image. The thermal transfer method has been acknowledged as a method which enables forming an image using digital data without using any treatment liquid such as a developer, and forming an image having high quality comparable to that of a silver salt photograph.
It has been recognized that it is important to give a thermal insulation function and a cushion function to the thermal image receiving sheet for obtaining a high quality image forming property in the recording system using the dye thermal transfer method.
For such problems, known is a method in which a foamed film having a function of thermal insulation and a function of cushion is pasted on a substrate and an image receiving layer is provided on the foamed film. In this method, however, the foamed film shrinks by heat generated when the image receiving layer is applied on the foamed film, resulting in curling of the produced sheet. In order to overcome the problem, various methods have been tested, for example: (i) using a novel functional layer having a thermal insulation function and a cushioning function; (ii) using a thermal transfer image receiving sheet produced by a process including no pasting step of the foamed film to minimize the curl caused by the heat generated in the production process; and (iii) using a variety of coating method.
For example, disclosed is a production method of a thermal transfer image receiving sheet in which an aqueous intermediate layer containing hollow particles and an aqueous image receiving layer containing a release agent are formed by a wet-on-wet coating method. Thus prepared thermal transfer image receiving sheet exhibits: (i) a sufficient adhesion of the image receiving layer and the intermediate layer; (ii) a satisfactory anti-blocking property against an ink-donor sheet; and (iii) a high drying energy efficiency in the production process (for example, see Patent Document 1).
However, in a replication test of Patent Document 1, the following features were observed:
(i) In a currently conducted high rate printing, the printing sensitivity is not fully enough. This is supposed to be because the colorant transferred from the thermal transfer ink sheet tends to have a hydrophobic nature, resulting in a relatively poor dyeing property of an image receiving layer containing a hydrophilic binder. Accordingly, a high flatness of the image receiving layer surface is required and a high coating accuracy become important. However, in the disclosed technique of Patent Document 1, the flatness is not fully sufficient and white spots in the image tends to occur. In a process of coating an intermediate layer containing low specific gravity particles, for example, hollow particles, followed by coating an image receiving layer on the intermediate layer, the image receiving layer is coated while the previously coated intermediate layer is still wet, as disclosed in the above documents. Therefore, the hollow particles often float on the surface of the intermediate layer, and the surface flatness tends to be lost. In a structure of an image receiving layer in which the intermediate layer is adjacent to the image receiving layer, the unevenness of the intermediate layer surface directly affects the flatness of the image receiving layer resulting in a tendency to give rise to white spots in the image;
(ii) The robustness of the adhesion between the intermediate layer and the image receiving layer is said to be one of the effects of the invention disclosed in Patent Document 1, however, it may have the following problem. Namely, in a high rate printing apparatus, in order to attain short time dyeing and the following short time release of layers, the amount of the release agent added to the image receiving layer is required to be increased. However, the increased amount of release agent may lower the adhesion between the intermediate layer and the image receiving layer;
(iii) Durability of an image may also be a problem. According to the current tendency to increase printing sensitivity, the molecular weight of a dye is becoming smaller, and the technique to prevent migration of dye at a higher temperature or under higher humidity is becoming important. However, in Patent Document 1, the provision to the above problem is not fully considered and lowering of hue density of an image or bleeding of the image tends to be observed at a higher temperature or under higher humidity, after formation of an image;
(iv) A wet-on-wet coating method may provide a higher drying energy efficiency, however, the number of coating steps are not reduced and the productivity of a thermal transfer image receiving sheet is not fully increased.
Patent Document 1: Japanese Patent Publication Open to Public Inspection (hereafter referred to as JP-A) No. 6-171240